This invention relates to a method of correcting uneven densities that occur in the image being recorded on a thermal recording material (hereunder referred to as a "thermal material") with a thermal recording apparatus in association with image data.
Thermal materials such as thermal films comprising a thermal recording layer on a film substrate are commonly used to record images produced in diagnosis by ultrasonic scanning. This recording method eliminates the need for wet processing and offers several advantages including convenience in handling. Hence in recent years, the use of the thermal recording system is not limited to s mall-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, the thermal recording apparatus uses a thermal head having a glaze in which heat generating resistors corresponding to the number of pixels in one line are arranged in one direction and, with the glaze a little pressed against the thermal recording layer of the thermal material, the thermal material is transported for example by transport means such as a transport roller to be relatively moved in a direction approximately perpendicular to the direction in which the heat generating resistors are arranged, and the respective heat generating resistors of the glaze are heated in accordance with the image data to be recorded to heat the thermal recording layer of the thermal material, thereby accomplishing image reproduction.
In the thermal recording apparatus, the force of friction at the interface between the running thermal material and the thermal head changes in accordance with the density of the image being recorded on the thermal material. For example, depending on its characteristics, the thermal material is insufficiently melted on the surface during low-density recording that its surface is not in a highly slippery condition. On the other hand, during high-density recording, the surface of the thermal material is sufficiently melted to become highly slippery.
As a result, at the boundary between two areas of the thermal material where the recording density experiences an abrupt increase, namely, at the transition of the surface of the thermal material from the less slippery state to a slippery state, the transport speed of the thermal material increases momentarily and only the recording density in the transition area will drop to cause unevenness in density in the form of white streaks. Conversely, at the transition from the slippery to a less slippery state, the transport speed slows down momentarily to cause unevenness in density in the form of black streaks.
This problem is discussed below in a more specific way.
FIG. 9 shows conceptually an example of the image being recorded. As shown, the image being recorded consists of a rectangular high-density area in the center of the thermal material and the surrounding low-density area. If the thermal material is transported in the direction of an arrow, the low-density area in the lower part of FIG. 9 is first recorded, then the central high-density area is recorded and finally the low-density area in the upper part is recorded.
The transport rollers, or rollers for transporting the thermal material are controlled by a transport motor such that the thermal material is transported at a constant speed at all times; however, as already mentioned, the force of friction between the thermal material and the thermal head will vary with the recording density, causing a change in the torque of the transport motor that is required to transport the thermal material. A comparatively large transport torque is required when the surface of the thermal material is less slippery but a comparatively small transport torque will suffice if the surface of the thermal material is slippery.
The transport rollers on the thermal recording apparatus are usually made of rubber and the shape of rubber rollers is deformed in response to the change in the transport torque. Briefly, the greater the transport torque, the more deformed the rubber rollers will be. Hence, the rubber rollers are deformed abruptly when recording is done at the transition from the area of small transport torque to the area of large torque; conversely, the rubber rollers will revert to the initial shape abruptly when recording is done at the transition from the area of large transport torque to the area of small torque.
In the illustrated case, if recording is done at the boundary between two areas of the thermal material where there is a transition from the low-density area in the lower part of FIG. 9 to the central high-density area, the transport torque decreases abruptly, whereupon the greatly deformed rubber rollers will revert to the initial shape so that the transport speed of the thermal material increases momentarily to lower the recording density, thereby producing a white line across the thermal material in a direction perpendicular to the direction of its transport. Conversely, a black line will develop if recording is done at the boundary where there is a transition from the central high-density area of the thermal material to the low-density area in the upper part.
Rubber rollers are used as the transport rollers in order to ensure that the thermal material being transported is depressed sufficiently uniformly to improve the precision in its transport, thereby producing a recorded image of high quality. Non-rubber rollers such as metal rollers are incapable of depressing the thermal material uniformly in the presence of slight distortions, hence failing to transport the thermal material in high precision. On the other hand, the use of rubber rollers has a limitation in that no matter how much improved the transport motor is in terms of performance, the image being recorded will experience the aforementioned unevenness in density.
Thus, the prior art thermal recording apparatus has had the problem that depending on the constituent material of the means for transporting the thermal material, uneven densities occur at density changing boundaries in response to the change in transport torque on account of the variation in recording density.
This reduction in the precision of image recording results in the deterioration of the quality of finished images and, particularly in medical areas where high-quality images need be recorded, the defect can potentially cause a serious problem by leading to a wrong diagnosis.